The present invention relates generally to pulse width modulated inverter circuits which can be used in current mode hysteresis controllers to control alternating current (AC) motors that are powered by direct current (DC) power sources such as batteries or rectified AC power through the inverter circuits and, more particularly, to a method of controlling such inverter circuits which results in improved low level torque control when used to control AC motors. While the present invention is generally applicable for controlling inverters for example to drive AC motors having any number of phases, it will be described herein with reference to the control of a three phase AC motor for which it is initially being applied.
AC motors as compared to DC motors are generally of simpler structure and include advantages of lower cost, more compact size, less weight and higher operating efficiencies. However, the complexity and expense of controllers for AC motors has hampered their adoption for use with DC supplies such as batteries and, in particular, for applications in motor vehicles. Simplification of AC motor controllers has resulted from the use of current mode controllers.
For current mode control, currents flowing in the motor are measured and compared to reference current signals which are generated by a motor controller and are representative of motor currents required to effect desired motor operating conditions. Error signals resulting from the comparisons are used to switch semiconductor elements of an inverter circuit which provides power to the motor. In this way, pulse width modulated (PWM) DC power is rapidly switched to control currents flowing in the motor and thereby substantially conform the motor currents to correspond to the reference current signals.
One popular form of current mode control uses hysteresis comparators. In hysteresis current mode control, the actual currents within a motor are maintained within a hysteresis band of given amplitude centered on reference currents which are representative of desired current levels within the motor. An example of hysteresis current mode control for use in a motor vehicle is disclosed in U.S. Pat. No. 5,027,048 which is assigned to the assignee of the present application and is incorporated herein by reference. The hysteresis motor controller of the referenced patent also discloses use of field oriented control which is another important improvement in the control of AC motors.
In hysteresis current mode control, if an actual motor phase current is less than the current defined by its reference current signal, and the difference or error signal is greater than half the hysteresis band, the inverter is configured to connect that motor phase to positive DC power to increase the current flow into the motor. Alternately, if the actual motor phase current is greater than the current defined by its reference current signal, and the magnitude of the difference or error signal is greater than half the hysteresis band, the inverter is configured to connect that motor phase to negative DC power to decrease the current flow into the motor. If the magnitude of the difference between actual and reference currents is less than half the hysteresis quantity, no change is made to the configuration state of the inverter.
Unfortunately, a problem occurs in hysteresis current mode controllers when the reference currents are small. If the reference currents are sufficiently small so that they are within the hysteresis band, all of the motor phases can be connected to either positive DC power or negative DC power which condition is referred to as a zero state. In a zero state, no external voltage is connected across the motor phases, an uncontrolled small current flows and an uncontrolled small torque is produced by the motor. Thus, for a zero state, while the inverter is technically operating properly, the torque produced by the motor is not what is being commanded by the reference current signals.
There is thus a need for improving the low level torque control in AC motors which are controlled by hysteresis mode current controllers.